Field of the Invention
The present disclosure relates to an organic light emitting diode display for improving image quality based on a result of sensing changes in driving properties of pixels.
Discussion of the Related Art
An active matrix organic light emitting diode (OLED) display includes organic light emitting diodes (OLEDs) capable of emitting light by itself and has advantages of a fast response time, a high emission efficiency, a high luminance, and a wide viewing angle. Each OLED includes an anode, a cathode, and an organic compound layer formed between the anode and the cathode. The organic compound layer includes a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL. When a driving voltage is applied to the anode and the cathode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the emission layer EML and form excitons. As a result, the emission layer EML generates visible light.
Each pixel of the OLED display includes a driving element controlling a current flowing in the OLED. The driving element may be implemented as a thin film transistor (TFT). It is preferable that electrical properties, such as a threshold voltage and a mobility of the driving element are equally designed in all of the pixels. However, the electrical properties of the driving TFT are not uniform due to process conditions, a driving environment, etc. As a driving time increases, a stress of the driving element increases. Further, the stress of the driving element varies depending on a data voltage. The electrical properties of the driving element are affected by the stress. Thus, the electrical properties of the driving TFTs vary as the driving time passed.
A method of compensating for changes in driving properties (or characteristics) of the pixels of the OLED display is classified into an internal compensation method and an external compensation method.
The internal compensation method automatically compensates for a variation in threshold voltages of the driving TFTs inside a pixel circuit. Because the current flowing in the OLED has to be determined irrespective of the threshold voltage of the driving TFT so as to implement the internal compensation, configuration of the pixel circuit becomes complicated. Further, it is difficult for the internal compensation method to compensate for a variation in mobilities of the driving TFTs.
The external compensation method senses the electrical properties (for example, the threshold voltage and the mobility) of the driving TFTs and modulates pixel data of an input image based on the result of sensing through a compensation circuit located outside a display panel, thereby compensating for changes in the driving property of each pixel.
The external compensation method directly receives a sensing voltage from each pixel through reference voltage lines (hereinafter referred to as “REF lines”) connected to the pixels of the display panel, converts the sensing voltage into digital sensing data to generate a sensing value, and transmits the sensing value to a timing controller. The timing controller modulates digital video data of the input image based on the sensing value and compensates for changes in the driving property of each pixel.
As a result of recent increase in a resolution of the OLED display and efficiency of an organic compound, an amount of current (or a current required per pixel) required to drive each pixel has also sharply decreased. Further, an amount of sensing current, which is received from the pixel so as to sense changes in the driving property of the pixel, decreases. As the amount of sensing current decreases, a charge amount of a capacitor of a sample and hold circuit decreases in a limited sampling period. Thus, it is difficult to sense changes in the driving property of the pixel. The sampling period is defined by a switching signal determining charge timing of the capacitor of the sample and hold circuit. During the sampling period, the sample and hold circuit receives current from the pixel, charges the capacitor with charges, converts the current into a voltage, and samples the voltage of the pixel.
The OLED display applies a low gray level sensing data voltage to the pixel, so as to sense the driving property of the pixel at a low gray level. In this instance, the OLED display converts current flowing in the pixel into a voltage through the sample and hold circuit, samples the voltage of the pixel, and converts the sampled voltage into digital data (i.e., the sensing value) through an analog-to-digital converter (ADC), thereby sensing the driving property of the pixel at the low gray level.
Because an amount of current of the pixel at the low gray level decreases, an input voltage of the ADC obtained in the limited sampling period may be less than a minimum voltage the ADC can recognize. If the input voltage of the ADC does not exceed the minimum voltage the ADC can recognize, the driving property of the pixel at the low gray level cannot be sensed. If a length of a sensing period including the sampling period increases, the input voltage of the ADC at the low gray level may increase. However, there is a limit to an increase in the length of the sensing period. If the driving properties of the pixels at the low gray level are not sensed, a variation in the driving properties of the pixels at the low gray level cannot be compensated. Because a current of the pixel at a high gray level become large, driving properties of high-resolution and high-definition pixels at the high gray level can be easily sensed.